1. Field of the Invention
This invention relates to an improved process for the conversion of hydrocarbons, and more specifically for the catalytic reforming of gasoline-range hydrocarbons.
2. General Background
The catalytic reforming of hydrocarbon feedstocks in the gasoline range is an important commercial process, practiced in nearly every significant petroleum refinery in the world to produce aromatic intermediates for the petrochemical industry or gasoline components with high resistance to engine knock. The widespread removal of lead antiknock additive from gasoline and the rising demands of high-performance internal-combustion engines, increasing the required knock resistance of gasoline components as measured by gasoline "octane" number, have been a major factor in the growth of catalytic-reforming capacity and continue this trend in many areas of the world. The market for petrochemicals derived from gasoline-range aromatics continues to grow substantially, creating a need for incremental reforming capacity, severity and/or efficiency. Many producers of aromatics are looking for ways to use or upgrade existing reforming capacity through more effective reforming processes and catalysts in order to meet this incremental need without building expensive new catalytic-reforming process units.
Catalytic reforming generally is applied to a feedstock rich in paraffinic and naphthenic hydrocarbons and is effected through diverse reactions: dehydrogenation of naphthenes to aromatics, dehydrocyclization of paraffins, isomerization of paraffins and naphthenes, dealkylation of alkylaromatics, hydrocracking of paraffins to light hydrocarbons, and formation of coke which is deposited on the catalyst. Increased aromatics and gasoline-octane needs have turned attention to the paraffin-dehydrocyclization reaction, which is less favored thermodynamically and kinetically in conventional reforming than other aromatization reactions. Considerable leverage exists for increasing desired product yields from catalytic reforming by promoting the dehydrocyclization reaction over the competing hydrocracking reaction while minimizing the formation of coke.
The effectiveness of reforming catalysts comprising a non-acidic L-zeolite and a platinum-group metal for dehydrocyclization of paraffins is well known in the art. The experimental use of these reforming catalysts to produce aromatics from paraffinic raffinates as well as naphthas has been disclosed by a number of companies active in technology development. Commercialization of this dehydrocyclization technology nevertheless has been slow, probably due at least in part to the reluctance of aromatics producers to spend large sums of money on entirely new processing units in order to expand capacity. The present invention represents a novel approach to the use of this technology in the context of an existing catalytic-reforming process unit.
Separation and recycle of a paraffinic fraction to reforming is disclosed in U.S. Pat. No. 2,853,437 (Haensel). Naphtha is split into low-boiling and a high-boiling fractions, with the low-boiling naphtha being combined with a high-boiling paraffinic recycle and the high-boiling naphtha combined with low-boiling paraffinic recycle. Each combined stream preferably is charged alternately to catalytic reforming, and the reformate is solvent-extracted to separate a paraffinic fraction; the paraffinic fraction is fractionated to obtain the low-boiling and high-boiling paraffinic recycle to reforming. U.S. Pat. No. 2,915,453 (Haensel et al.) teaches reforming of naphtha and heavy paraffinic raffinate, separating a raffinate from the reformate by solvent extraction, and fractionating the raffinate to recover a low-boiling paraffinic fraction which is sent to a separate reforming zone.
U.S. Pat. No. 3,001,928 (Grote) discloses reforming of a gasoline fraction, subjecting the heavier reformate product to solvent extraction to recover a non-aromatic raffinate, and reforming the raffinate. The raffinate is reformed at a pressure at least 75 pounds per square inch lower than that at which the gasoline fraction is reformed.
Selective adsorption of multibranched paraffins from a feed mixture also containing singly-branched and/or normal paraffins using a crystalline aluminosilicate is disclosed in U.S. Pat. No. 3,706,813. The zeolite must contain a certain percentage of water, and preferably is X or Y zeolite modified with barium cations. U.S. Pat. No. 5,107,052 teaches the adsorptive separation of dimethylparaffins from an isomerate comprising C.sub.4, C.sub.5 and C.sub.6 hydrocarbons using a zeolite or aluminophosphate isostructural with AIPO.sub.4 -5.
U.S. Pat. No. 4,648,961 (Jacobson et al.) discloses dehydrocyclization of a naphtha feedstock with a monofunctional large-pore zeolite catalyst, separating normal and single-branched paraffins from the aromatics product, and recycling the paraffins to the dehydrocyclization step. The paraffins are extracted for recycle using a molecular sieve, which adsorbs normal paraffins and some of the isoparaffins.